Multiplex radio communication



June 20, 1933.

c. EJE NKINS MULTIPLEX RADIO COMMUNICATION Filed Sept. 1, 1926 Fig.1.

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s Sheets-Sheet 1 Patented June 20, 1933 UNITED STTES FATE a CHARLESFRANCIS JENKINS, OF WASHINGTON, DISTRICT OF COLUMBIA, ASSIGNOR TOJENKINS LABORATORIES, OF WASHINGTON, DISTRICT OF COLUMBIA, A CORPORA-TION OF THE DISTRICT OF COLUMBIA MULTIPLEX RADIO COMMUNICATIONApplication filed September 1, 1926.

My present invention relates to a signalling system and moreparticularly to asystem for transmitting and receiving a plurality ofsignals simultaneously.

- An object of my invention is to provide a system in which a pluralityof signals (or messages or other communications) may be transmitted on asingle carrier wave.

l/Vith this object in view my invention contemplates an arrangement inwhich a carrier wave is alternately modulated by signals from twoseparate sources. the rate of alternation being above audibility. At thereceiving station the carrier wave is received, detected and the twomessages are separated and supplied to separate receivers.

More particularly my invention contemplates an arrangement in which ahigh frequency carrier wave is modulated by" an intermediate frequencywave, the positive alternations of which are modulated in accordancewith one signal, while the negative alternations are modulated inaccordance with the other signal. Preferably the carrier wave ismodulated by the two signals in opposite senses, that is to say, onesignal effects a reduction in the amplitude of the carrier wave alone,while the other signal effects an increase in the amplitude of theiarrier alone. At the receiving station the carrier wave is received anddetected to derive therefrom the intermediate frequency waves, thepositive and negat1ve alternations of which bear the original messagevSerial No. 132,979.

My present invention is illustrated in the accompanying drawings, inwhich- Figure 1 is a circuit diagram of one form of the arrangementemployed at the transmitting station;

Figure 2 is a circuit diagram of one form of receiving apparatus whichmay be employed;

vFigure 3 is a circuit diagram of a modified transmitting apparatus;

Figure 4 is a circuit diagram of a modified form of receivingarrangement;

Figure 5 is a characteristic curve useful in explaining the operation ofthe invention;

Figures 6 and 7 are wave diagrams employed in explaining the operationof the transmitting apparatus.

In Figure 1, part 0 is an ordinary threeelectrode, hot-cathode, electrontube which, with its associated circuits, constitutes a generator of anintermediate frequency ourrent. The frequency of this current is pref-,

er'ably above audibility, for example, it may be 60 kilocycles. However,I do not intend to be limited to any particular value of frequency.Oscillator supplies intermediate frequency currents of the same phase totwo secondary windings S and Sarranged respectively in the inputcircuits of amplitiers M and M The output circuits of amplifiers l 1 andM are coupled respectively to the input circuits of rectifier-modulatorsM and M the output circuits of the modulators being connected in amanner to modulate the current generated by the high frequencyoscillator 0 Two signailing devices C and C are connected respectivelythrough amplifiers A and A to the input circuits of additionalamplifiers M and M The signalling devices shown in Figure 1 arepreferably light-sensitive cells, although it is to be understood thatfor certain purposes other types of signalling devices, such as amicrophone, or a telegraphic signalling device, may be used instead ofthe light-sensitive cells. The light sensitive cells may be either ofthe type in which an internally generated electromotive force is variedin accordance with the light falling on the cell, or of the type inwhich the internal resistance of the cell varies in accordance with thelight variations. Batteries B and B are inserted in series with thecells in local circuits, includ ing resistance R and R respectively, forthe purpose of-energizing the cells. The input circuits of amplifiers Aand A are connected respectively across resistance R and R in the localcell circuits, the output circuits of amplifiers A and A being coupledrespectively to the input circuits of amplifiers M and M? by means ofcoupling resistancesR and R Resistances R and R are shunted by suitablecondensers for the purpose of by-passing the intermediate frequencycurrent. The grid element of rectifier-modulator M is normallymaintained at a negative potential by battery B, and the grid element ofrectifier-modulator M is normally maintained at a positive potential bybattery I3. T hesc potentials are adjusted to such values thatrectifier-mmlulator M operates at a point on the lower part of thecharacteristic curve and rcctiliermodulator M operates at a point on theup per part of the characteristic curve, the respective points beingindicated at M and. M on the characteristic curve, shown in Figure 5.The output circuits of modulators M and M are connected in parallel withthe plate circuit of the oscillator a radio-frequency choke-coil C beingint-erposed between rectifier-modulators and the oscillator O for theobvious purpose of preventing radio-frequency currents from O passinginto the rectifier-modulator.

Parts X and X are radio-frequency choke-coils (of a value depending onthe frequency of O placed in the plate circuit to provide a constantcurrent supply to the rectifier-modulators and the oscillator 0obviously these coils are necessary to modu- 'late 0 with the output of0 Batteries B and B supply the plate currents of reetifier-modulators Mand M and oscillator 0 It is obvious that asingle batteny may be used asa source of plate currentinstead of separate batteries I3 and B It is tobe understood that vacuum tube 0 with its associated circuits,constitutes a generator of high-frequency oscillations which may beradiated from an antenna as shown in Figure l, or as is well known thesehigh. frequency currents may be transmitted over line wires to 'thereceiving station. The frequency of the "are generated by oscillator 0may be of the order of 3000 kiloc scles, although I do not intend to belimit-ed to any particular frequency.

The operation of the embodiment of Figure 1 i s as follows:

Assuming for the present that oscillator O is not operating, oscillatorO generates a high ir-ouency current of constant amplitude representedby the curve CW in Figure 7, the umnodulated portions of the currentbeing represented between vertical lines III and IV, and V and VII Itwill be understood that under normal conditions current from batteriesI3 and B is dis tributed among the plate circuits ofrectificr-nmdulators M, M" and oscillator 0 in such proportion thatoscillator 0 receives a plate current of a certain normal value,rectifier-nuidulator M receives a plate current of high value, indicatedat point M in Figure 5, and modulator M receives very little platecurrent, indicated at point M in Figure Assuming now that oscillator isin operation, intermediate frequency currents having the same phase andfrequency will be supplied to amplilicls M and M", which will induceinto the input circuits of rcctifier-modulators M and M i ntcrmcdiatefrequency currents having the same phase and frequency. Reicrring toFigures (3 and '7, the vertical lines indicated by the numerals I to VIIindicates the half-cycle divisions of the alternating currcnt' generatedby oscillator 0*.

Line L" is an ori in line for rectifier-modulator M and line L is anorigin line for current is normally very low, but since modulator M" isbiased by a positive potential the etl'eet of this negative modulatingpotential is to increase the lilament-to-plate resistance of the tube,which results in a decrease in the plate current of modulator hi and acorresponding increase of plate current in oscillator The increasedplate current supplied to oscillator 0 1esults in an increase in theamplitude of the high frequency waves indicated between lines I and IIinFigure 7. Assuming next that a positive alternation of theintermediate frequency waves is impressed upon the grid elen'ients ofrectifier-modulators M" and M", it will be seen from Figure 5 that theplate current in rectifier-modulator M" will be practically unat cctedsince due to the high positive bias, this tube is normally working atthe saturation point, but in case ol reeliliws-unululator M the positivealternation will act to oppose the negative bias and thereby decreasethe filament-to-plate resistance, with the result thatrectifier-modulator M will take an increased plate current. The decreasein resistance of rectifier modulator M will rob the plate circuit ofantern oscillator O of some of its current which will result in adecrease in the amplitude of the high frequency waves indicated be tweenlines H and HI Figure 7 and be tween lines IV and V of the same Figure.It will readily be understood that the ant plitude of the positive andnegative alter 'nations of the intermediate frequency wave supplied torectifier-modulators M and M are controlled respectively by thesignalling devices C and C The wave which is radiated is a doublymodulated wave, that is, a high frequency modulated by an interme diatefrequency wave which in turn is modulated by signal modulations, thepositive alternations of the intermediate frequency wave being modulatedin accordance with one signal, while the negative alternations aremodulated in accordance with another signal.

The receiving arrangement employed for separating the two signals ormessages is shown in Figure 2, in which D is a vacuum tube detector, theinput circuit of which is coupled to a suitable receiving antenna andthe output circuit ot which is coupled to a vacuum tube amplifier A bymeans of a coupling resistance It. The output circuitol amplifier A iscoupled to the input circuit of amplifier A by coupling resistance Theoutput circuit of amplifier A is coupled to a double detectorarrangement comprising two vacuum tube detectors D and D two signalindicating devices T and T and a common plate battery B". The indicatingdevices T and T are preferably audible signal indicators, for example,telc phones, buzzers, or telegrapl'iic sounder-s, visual or lightindicators or any combination of these, and are connected respeciivel inthe plate circuits of detectors D and D. The input circuits of detectorsD and D are coupled to the output circuit of amplifier A across couplingresistances R and B respectively. Each input circuit con tains a gridbiasing battery, indicated at With respect to the output circuit ofampli- B and B", The potentials of these batteres are so chosen thatnormally no current flows through the signal indicating devices T and TIt Will be seen that detectors l) and l)" are connected in adifferential arrangement fier A, that is, they are so connected tl 1,the drop in potential across the coupling resistance will produce anegative biasing voltage in one grid circuit and a positive biasingvoltage in the other grid circuit.

Normally the combining voltage of batteries B and B and drop acrossresistances R and B respectively are so adjusted that the plate currentof detectors D and D is reduced substantially to zero. The polarity ofthe voltage drop across each resistance is'indicated by appropriatesigns in the drawings.

The operation of the receiving arrangement is as follows:

The incoming Wave is received and detected by detector D to derivetherefrom the intermediate frequency Wave bearing the original signalmodulations, the positive alternations bearing the modulations of onesignal and the negative alternations bearing the modulations of theother signal. The intermediate frequency Wave is amplified successivelyby amplifiers A and A. and fed into the double detector arrange mentcoupled to the output circuit of amplifier Assuming that a positivealternation of the intermediate "frequency wave is flowing in the outputcircuit of amplifier it. and turther assuming that the positivealternation te is to increase the normal plate current and producecorresponding increase in voltage drop across resistances R and it inthe input circuits of detectors D and D respectively, the increase involtage drop across resistance R has the eitect of rendering the gridmore negative than normally and, therefore, there will be no change incurrent and no response in signal indicator T but since the voltage dropacross resistance R tends to oppose the negative bias on the grid of thedetector; D a. plate current will flow and produce a response in signalindicator T Assuming next that a negative alternation of theintermediate frequency wave is impressed upon the output circuit ofamplifier A, the result will he a decrease in the plate current of theamplifier. The decrease in the plate current of the amplifier A willagain unbalance the biasing voltages in the detector circuits, but in anopposite manner, with the result that plate current will flow andproduce a response in signal indicator T and there will be no responsein indicator T Thus it will be seen that the receiving arrangement isselective With respect to the positive and negative alternations of theintermediate frequency Wave, andthe separate messages may be receivedindependently upon indicators T and T From the foregoing it will be seenthat the action of the receiving arrangement is that of a high-speedswitching device for (uteinatically and instantaneously switching thesuccessive positive and negative alternations, bearing the differentsignals, into their respective signal detecting and indieating cuits. Itis obvious that this orraireincm. has general utility, and maybe invarious situations Where it is desires to separate positive and negativepulsations a single channel and divert them into individual channelsjThe modified term of transmitting apparatus shown in Figure 3 dititersfrom the arrangement of Figure 1, in that a separate intermediatefrequency oscillator is provided for each signalling device C and C Thearrangement for modulating the intermediate frequency waves by thesignalling currents is also slightly different. Where elements serve thesame function in the two figures they have been indicated by the samereference characters. Parts 0 and with their associated circuits,constitute separate generators of intermediate frequency current. Thesetwo oscillators should be maintained in synehronism and in proper phaserelationship, and to accomplish this the 0scillators are controlled froma common piezo-electric resonator.v This resonator comprises apiezo-electric crystalv Q, preferably of quartz, having mounted on oneside thereof two separated conducting plates (1 and I), and a thirdconducting plate 0 on the opposite side. Plate a paired with c isconnected across the frequency determining circuit of oscillator 0 whileplate I) also paired with c is connected across the frequencydetermining circuit of oscillator O. The dimensions of crystal Q are sochosen ,that it has a natural frequency equal to the intermediatefrequency. It will readily be understood that if the frequencydetermining circuits of oscillators O and O are adjusted approximatelyto the frequency of crystal Q, the action of the crystal, when set inoperation, will be to maintain'the oscillators in synchronism and properphase relationship. Parts M and M are modulators for modulatingrespectively the current generated by oscillators O and O in accordancewith the signals impressed upon signaling devices C and C As will beseen from the drawings the plate circuits of modulator M and oscillator0 are connected in serial relation with a single battery B for supplyingthe plate current. Similarly the plate circuits of modulator M andoscillator O are connected in serial relation, with battery 13, as asupply for plate current. The output circuits of amplifiers A and A arecon led res ectively to the input circuits of modulatorsM and M bymeansof coupling resistances 1 and R".

The operation of the embodiment of Figure 3 is as follows:

' Varying signals impressed upon the signalling device C and C producecorrespondingly varying currents in the output circuits of amplifiers Aand A. The potentials on the grids of modulators M and M will vary inaccordance with the varying plate currents of amplifiers A and A,thereby resulting in corresponding variations of the filament-to-plateresistances of modulators M and M The variations in internal resistancesof modulators M and M will produce corresponding changes in the platecurrent of oscillators O and 0 thereby modulating the current generatedby the oscillators in accordance with the signalimpressed upon theirrespective signalling device. It will be seen that intermediatefrequency currents having the same phase and frequency will be suppliedto the input-circuits of rectifier-modulators M and M", the currentsupplied to rectifier-modulator M being modulated in accordance withsignals impressed upon signalling device C From this point on, theoperation is the same as in the arrangement shown in Figure 1 and willbe readily understood.

A modified form of receiving apparatus is shown in Figure 4, whichdiffers from that shown in Figure 2 merely by the. substitution ofcoupling transformers L L and L for coupling resistances R, R, and R RThe operation of this form of receiving arrangement will be readilyunderstood in view of the foregoing description of the operation of thearrangement of Figure 2.

From the foregoing description it is apparent that the output ofmodulators M and M' constitutes a composite wave of the same frequencyas the intermediate frequency waves from which it is derived, andthatthe positive alternations of this composite wave are modulated inaccordance with one signal and the negative alternations are modulatedin accordance with another signal. It is therefore, obvious that insteadof modulating oscillator 0 directly from modulators M and M", thecomposite wave produced by these two modulators may be passed through aresistance coupled, intermediate frequency, power amplifier, in theusual manner, which would then modulate oscillator 0 If line wires areused as a transmitting medium, the composite wave produced by modulatorsM and M need not be transmitted by the carrier wave generated byoseillator but may be impressed directly upon the line and transmittedto a distant receiving station. In this arrangement a detector at thereceiving station will not be necessary, and the input circuit ofamplifier A would be connected directly to the line circuit.

It is also apparent that, if desired, amplifier M and M" in Figure 1 maybe omitted. In such an arrangement the output circuit of oscillator 0would be coupled directly to the input circuits of modulators M and M,and the output circuits of amplifiers A and A would be coupledrespectively to the input circuits of modulators M and M in the samemanner as they are shown connected to modulators M and M Also, it willthus be observed that I have provided a system including both method ofand apparatus for transmitting and receiving multiple signalsrepresenting any code, music or speecl1,-vi sual eflect's'oraudibleeffects. I

Having thus described my invention and illustrated its use what I claimas new and desire to secure by Letters Patent is:

1."' The' method of signaling which comprises generating a sustainedhigh frequency wave, diverting said wave into two separate channels,recombining the Waves in said channels to produce a single compositewave having portions above zero amplitude modulated only in accordancewith one signal, and. portions below zero amplitude modulated only inaccordance with another signal.

2. In a signaling system, the combination of-means for generating asustained high frequency wave, means 'for modulating only the positiveportion of the Wave by one signal,;and for modulating only the negativeportion ofqthe. wave by another signal, means to ieceivesaid modulatedwaves, means to separate thejalternate modulated'portionsjof; h ave,landmeans to detectjsaid modulated Wave. portions to derive therefromthe's'eparate: signals;-

n'alin'g which v 3. A method of multiplex-sig comprises,regenerating a'carrier-Jin the form v of sustained oscillations having a frequency[above audibility, modulating .the positive alternations of saidWave-in.accordancefw-ith one signal, modulating .the negative{alternations in accordance withia second signal, transmitting the.modulated wave to a'receiving stationydiverting the positive. and] Iflonly of whlch'are modulated in accordance With one signalyandthenegative alterna- "tions only iof whi'chflare modulated in ac 4.In a signaling system the combination negative alternations intoseparate-channels, and detecting [separate signal indications from said,alternatmns;

of, a signaling channel, means 'tofimpress upon said channel aninaudible carrier n the form of, sustained. oscillations, means tomodulate the positive alternations :of said carrier in accordancewith-one signal and means to modulate thenegative' alternations inaccordance with another signal, means for transmitting both saidmodulated alternations to a receiving station, means at, the receivingstation ,for diverting the modulated positive and negative alternationsfrom said channel into separate channels, and means in each of saidseparate channels to detect the signal transmitted.

5 .v A method of signaling which comprises generating'a carrier in theform of sustained oscillations, modulating the positive alter nations ofsaid carrier in accordance with a signal, modulating the negativealternations signal, generating a second carrier, and modulating saidsecond carrier by the first modulated carrier, transmitting the secondmodulated carrier to a distant receiving station, receiving anddetecting said resultant Wave to derive therefrom the original firstcarrier, and producing independent signal indications by the positiveand negative alternations of said derived carrier.

6. In a signaling system the combination of, a signaling channelcarrying a superaudible current having positive and negative portionsindepclnlently modulated byseparate signals, and means associated, withsaid channel for diverting the positive'and negative portions of thecurrent into separate channels, said means comprising a pair of threeelectrode electron discharge devices.

7. In a signaling system the combination of, a signaling channelcarrying a superaudible wave bearing modulations in accordance withseparate signals, and means for separating and diverting the modulatedportionscorresponding to the separate signals into separate" channels,said means including azpairoi' detectors oppositely biased by;.the'odulated wave.

8. In a "signalln'g system the combination of, a source. of Waveshavingportions above and below Zero amplitude respectively modulated inaccordance with diiferent signals, means for diverting the respectiveportions of each signal into] separate channels and means 111 eachchannel responsrveto the re;

spective wave portions foi -i, producing separatesignals, the lastmentionedmeans 111- eluding a pair of difi'erentially connected electrondischarge devices. jj-

9. In a signaling system the combination of, a source of alternatingcurrentolf superaudible frequency, the positivealternationscordanceiwithf asecond signal, and. means"associ'ate'd'fwithisa-idfisource for diverting the positive andnegative alternations into: sepchannels and .nieans in each channelresponsive to the alternations for-producing .isep'arate signals, .thelast mentioned means including'a pair of electron discharge devicesoppositely biased by the modulated alternations.

10. The method of signaling which comprises generating a carrier currentin the form of sustained oscillations of normally constant amplitude,modulating alternately succeeding sections of said wave in accordancewith separate signals to be transmitted from said station, and causingone of said signals to increase the am litude of said wave and the otherof sai signals to decrease the amplitude of said wave.

11. The method of signaling which com- 65 otsaid carrier in accordancewith a second prises, generating at a transmitting station a carriercurrent in the form of sustained oscillations, modulating alternateportions of said current in accordance with separate messages, onemessage effecting only an increase in the amplitude of said carrier, andthe other message effecting only adecrease in the amplitude of saidcarrier; y

12. In a signaling system the combination of afirst source of sustainedoscillations, means for dividing the current from said source into twoseparate channels, a signaling device associated with each channel formodulating both halves of the oscillatlng waves in the said channels, arectifier-modulator associated with each channel, saidrectifier-modulators being biased in oppositesenses so that one passescurrent only above a certain selected value and the other passes currentonly below said certain selected value, a common output circuit for saidrectifier-modulators, a second source of sustained oscillations, andmeans for modulating the oscillations from said second source inaccordance with the currents in said common output circuit.

13. In a signaling system the combination of means for generating asustained high frequency wave, means for diverting said wave into a pairof separate channels, and means including separate signaling devices fortranslating said diverted waves into a single composite wave, saidcomposite wave having a portion above zero amplitude modulated only inaccordance with one of said signal devices, and the remaining portion ofsaid composite wave below zero amplitude modulated only in accordancewith said second signal device.

14. A radio transmission system comprising a generator of alternatingcurrent of.

carrier frequency above audibility, and

v means for modulating either the positive or the negative componentofthe alternating current independently of the other half.

15. A communication system comprising means for producing andtransmitting an alternating current of carrier frequency aboveaudibility with either the positive component or the negative componentor each of said components modulated independently of the other, and areceiving apparatus comprising two detectors, one detecting the positivecomponent of the received carrier wave and the other detectingthenegative component thereof.

16. A radio transmission system comprising a generator of alternatingcurrent of carrler frequency above audibility, and means for modulatmgthe positive and negative components of the alternating current inde- Imeans for modulating independently and successively the positive andnegative components of the alternating current.

18. A radio transmission system comprising a generator of alternatingcurrent of carrier frequency above audibility, an an tenna'to beenergized thereby, a rectifier interposed between said generator andsaid antenna for passing current pulsations in one direction onlybetween said antenna and generator, and means for modulating the currentpulsations passed by said rectifier.

19. A radio transmission system comprising a generator of alternatingcurrent at carrier frequency above audibility, an antenna to beenergized thereby, a pair of oppositely conductive rectifiersconstituting parallel paths for current pulsations in their respectivedirections of conductivity between said generator and said antenna, andmeans for modulating the current pulsations in each rectifierindependently of the other.

20. Method of modulating alternating current of carrier frequency aboveaudibility which comprises resolving the alternating current intopositive and negative components, and superimposing a modulation uponone of said components independently of the other.

21. Method of modulating alternating current of carrier frequency aboveaudibility which comprises resolving the alternating current intopositive and negative components, and superimposing a modulation uponeach of said components independently o the other.

22. Method of communication'which comprises resolving an alternatingcurrent of carrier frequency above audibility into positive and ne ativecomponents, superimposing a modu ation upon one of said componentsindependently of the other and transmitting the component thusmodulated.

23. Method of communication which comprises resolving an alternatingcurrent of carrier frequency above audibility into positive and negativecomponents, superimposing a modulation upon each of said componentsindependently of the other, and transmitting the two components thusindependently modulated as a complete carrier wave.

24. A communication system comprising means for producing andtransmitting an alternating current of carrier frequency aboveaudibility with either the positive component or the negative componentor each of said components modulated independently of the other, andareceiver having selective means for detecting either the positive orthe negative component, at will.

25. The method of signaling which comprises generating a sustained highfrequency wave, modulating only the positive alternations of said wavewith a signal to be propagated and modulating only the negativealternations of said wave With a different signal to be propagated.

26. The method of signaling which com- I prises generating a sustainedhigh frequency wave, separably modulating the positive and negativeportions of said wave with different signals, transmitting the modulatedportions as a composite carrier wave to a receiving station, conductingone-half portion of the composite wave through a rectifier and detectingand indicating the modu, lations in said portion of the wave passingthrough said rectifier.

27. A high frequency transmitter comprising a source of continuous wavehigh frequency oscillations, means for propagating said oscillations, arectifier in circuit with-said source, a signal device for modulatingsaid oscillations independently of said rectifier said rectifier beingconductive to one half portion only of the high frequency wave from saidsource, and means including a second signal device operable to modulatein accordance with signals to be transmitted only that half wave towhich said rectifier is conductive.

28. A high frequency transmitter comprising a source of continuoussuper-audible waves, a pair of signaling generators, a pair ofasymmetrically conductive vacuum tubes in circuit with said source,,saidvacuum tubes being arranged so as to be conductive to the two halfportions respectively of the high frequency wavesfrom said source, andmeans including said generators for independently varying theconductivity of the respective circuit paths in which said vacuum tubesare individually included, said means being operable to effectmodulation of the two half port-ions of the high frequency Wave, eachindependently of the other.

29. A high frequency wave signaling system comprising a source ofsuper-audible waves, means for modulating the two por tions of said waveabove, and below zero amplitude independently, means for transmittingthe modulated waves, and a receiving station comprising an asymetricallyconductive path operable to pass one half of said modulated wave andexclude the other half, and receivingapparatus at said receiving stationoperatively associated with said asymmetrically conductive .path.

In testimony whereof I have hereunder signed my name.

CHARLES FRANCIS JENKINS.

portion of the high frequency

